IMO it would be good to familiarize yourself with the rust quirks/features (check out the rust book) and then poke at some of the embedded specific groups. Once you get a handle on the common types and patterns it's probably easier to find the information you're looking for, e.g.:

https://github.com/rust-lang/rust/issues/46213

In general Rust chases a "zero cost abstraction" ethos, so a lot of the type system niceties have no code or memory cost because the heavy lifting is done at compile time, not run time. For instance using specific traits for each GPIO pin ensures you won't accidentally apply some meaning meant for another pin, but the compiler will compile it right down to integers anyhow.

Things like options (how Rust deals with "null" values) are enums and usually incur a one byte penalty, but in some cases they can be compiled down to no overhead at all.

> it's a bad comparison and I don't understand why

Different languages are different, and so it's tough to compare. You don't generally write Rust code in the same way you write C, and so a "using the same design" constraint on a comparison means that some folks will not consider it to be an accurate one.

In general, similar constructs have the same amount of overhead, that is, zero. Rust has additional features that are useful that can increase code size if you're not careful, but getting that back down doesn't necessarily mean abandoning Rust, but it may lead to "write the Rust closer to C." I am thinking of generics specifically here, monomorphization can blow up your code size, but you could either not use generics or use a few techniques to minimize it.